Conventionally, there is an optical distance measuring apparatus which uses a triangulation method to obtain a distance to an object to be measured in a non-contact manner as its measurement principle. This distance measuring apparatus, as shown in FIG. 1(b), for example, installs a camera 20 in the vertical direction to a reference surface of an object 3 to be measured, irradiates the reference surface S of the object 3 to be measured with a laser beam Bi at a prescribed light projection angle θi, and detects the change in a position of the surface of the object 3 to be measured in the vertical direction by the camera 20, as the change in a light receiving position of the reflected light of the laser beam Bi, and thereby obtains a distance of the object 3 to be measured from the position of a reference surface S.
Generally, as the camera 20 used in this case, an area scan type CCD camera, and a line scan type CCD camera in which an optical axis of a laser beam and an optical axis of the camera 20 for light receiving are set so as to be on the same plane are used.
The resolution of such an optical distance measuring apparatus using a triangulation method becomes a value which is obtained by dividing a distance measurement range D of the camera 20 by the number of bits of imaging elements of the CCD camera 20, and accordingly, the measurement range and the resolution are in a trade-off relation.
For this reason, there is an art which irradiates different distance measurement ranges with two laser beams, and discriminates the detection positions of the respective laser beams, to thereby try to expand the measurement range (Refer to Patent Document 1, for example.).